The present invention relates to an optical fiber amplifier and, in particular, to a method for varying an overall scale factor of the gain spectrum of the optical fiber amplifier without substantially changing the shape of the gain spectrum. Thus, while in a conventional erbium doped fiber amplifier (EDFA), the magnitude and shape of the gain spectrum are completely coupled, the present invention provides a technique for decoupling the magnitude and shape of the gain spectrum of an EDFA.
Achieving a uniform or flat gain spectrum is desirable for optical amplifiers used in wavelength-multiplexed communication systems. The gain flatness of an EDFA can be optimized by using host glass materials for the erbium that produce flat gain spectra (e.g., aluminum codoped silica or fluoride glasses such as ZBLAN) and operating the amplifier at an average inversion that provides optimally flat gain in the spectral region of interest (C. R. Giles and D. J. D. Giovanni, xe2x80x9cSpectral dependence of gain and noise in erbium-doped fiber amplifiers,xe2x80x9d IEEE Photonics Technology Letters, vol. 2, pp. 797-800, 1990). The gain flatness can further be improved through the use of gain flattening filters (M. Tachibana, R. I. Laming, P. R. Morkel, and D. N. Payne, xe2x80x9cErbium-doped fiber amplifier with flattened gain spectrum,xe2x80x9d IEEE Photonics Technology Letters, vol. 3, pp. 118-120, 1991). All of these techniques, however, only provide the optimum gain flatness at a single gain value (i.e., gain at any particular wavelength). It is well known that if the gain of an EDFA is changed by changing the inversion (e.g., by changing the relative pumping rate), the gain changes in a well defined spectrally dependent manner (C. R. Giles and D. J. D. Giovanni, xe2x80x9cSpectral dependence of gain and noise in erbium-doped fiber amplifiers,xe2x80x9d IEEE Photonics Technology Letters, vol. 2, pp. 797-800, 1990; J. Nilsson, Y. W. Lee, and W. H. Choe, xe2x80x9cErbium doped fibre amplifier with dynamic gain flatness for WDM.,xe2x80x9d Electronics Letters, vol. 31, pp. 1578-1579, 1995). As a result, if a conventional EDFA is used in an application where its gain needs to be different from the flattest gain of the amplifier, its gain spectrum will show excess normalized gain ripple ((maximum gainxe2x80x94minimum gain)/minimum gain as calculated on the wavelength band of interest).
An example of how this can be a problem is provided by an optically amplified fiber transmission system where one needs to support fiber spans shorter than those for which the amplifier is designed. It is typically impractical to have separate amplifiers custom designed for each fiber span. Therefore, one is either forced to have an amplifier with a distorted gain spectrum or to add enough loss to the system that the design gain is actually needed from the amplifier. The latter uses more optical power than a redesigned amplifier would and has inferior noise performance, even if the loss is added between stages of an EDFA (Y. Sugaya, S. Kinoshita, and T. Chikama, xe2x80x9cNovel configuration for low-noise and wide-dynamic-range Er-doped fiber amplifier for WDM systems,xe2x80x9d in Optical Amplifiers and their Applications, 1995 OSA Technical Digest Series, Vol. (Optical Society of America, Washington, D.C.) 158-161).
It is an object of the present invention to provide a novel technique which enables one to change an overall scale factor of a gain spectrum of an optical amplifier such as an EDFA without substantially changing the shape of the gain spectrum. More generally, it is an object of the invention to provide a technique for decoupling the magnitude and shape of the gain spectrum of an EDFA or other optical amplifier. (The two are completely coupled in a conventional EDFA.) Thus, using the present invention, it is possible to provide an EFDA in a span which is shorter than the design span without adding attenuation, by controlling the scale factor of the gain spectrum.
Gain can be obtained in an EDFA when some fraction of the erbium dopant ions are excited into the metastable 4I13/2 state. A signal band photon (wavelength typically 1525-1600 nm) incident upon an excited ion can stimulate the release of a photon identical to itself and cause the erbium ion to return the 4I15/2 ground state. Silica based EDFAs used in telecommunication systems are typically pumped at wavelengths near (+/xe2x88x92xcx9c25 nm) 1480 nm or 980 nm. As shown in FIG. 1, the former directly excites the ions from the ground state to the metastable level, whereas the latter makes use of the auxiliary 4I11/2 state. Other pump bands are possible, but lower power conversion efficiency is typically found in 650 nm and 800 nm bands due to excited state absorption (ESA) and consequently they are not used in commercial systems.
The gain spectrum of an EDFA can be approximately written as
GdB(xcex)=10 log10(e)xcex93(xcex)LEDF[{overscore (N2+L )}"sgr"e(xcex)xe2x88x92{overscore (N1+L )}"sgr"a(xcex)]xe2x80x83xe2x80x83(1)
where GdB is the amplifier gain in decibels, LEDF is the total length of the erbium-doped fiber (EDF) within the amplifier, "sgr"e and "sgr"a are the emission and absorption cross sections, respectively, and N1 and N2 are average populations (ions per unit volume) of the ground state and metastable levels, respectively (C. R. Giles and E. Desurvire, xe2x80x9cModeling erbium-doped fiber amplifiers,xe2x80x9d Journal of Lightwave Technology, vol. 9, pp. 271-283, 1991). The local fractional average inversion (i.e., inversion averaged over a cross section of the fiber at a particular axial point) is calculated as                                           n            i                    _                =                              1            L                    ⁢                                    ∫              0              L                        ⁢                                          ⅆ                                  z                  xe2x80x2                                            ⁢                                                ∫                  0                                      doping                    ⁢                                          xe2x80x83                                        ⁢                    radius                                                  ⁢                                  r                  ⁢                                      ⅆ                    r                                    ⁢                                      xe2x80x83                                    ⁢                                                                                    N                        i                                            ⁡                                              (                                                  r                          ,                                                      z                            xe2x80x2                                                                          )                                                              /                                          xe2x80x83                                        ⁢                                          [                                                                                                    N                            1                                                    ⁡                                                      (                                                          r                              ,                                                              z                                xe2x80x2                                                                                      )                                                                          +                                                                              N                            2                                                    ⁡                                                      (                                                          r                              ,                                                              z                                xe2x80x2                                                                                      )                                                                                              ]                                                        ⁢                                      xe2x80x83                                    ⁢                                                            {                                                                        i                          =                          1                                                ,                        2                                            }                                        .                                                                                                          (        2        )            
which will be convenient for re-writing the form of Eqn. (1) below.
EDFAs are typically used such that nearly all erbium ions are in the metastable 4I13/2 level (level 2) or the ground state 4I15/2. This is because efficiency reducing ESA is likely to be a problem if this is not the case. When pumping in the 980 band, ions are predominantly moved from the ground state to the 4I11/2 level. In silica this state has a short lifetime on the order of 10 xcexcs and the ions undergo non-radiative decay to the metastable state. Because the lifetime of the 4I11/2 level is so much shorter than that of the metastable (at the power levels typically encountered in commercial communication systems) the population of this level is typically negligible. In low phonon energy glasses (i.e., glasses with a phonon energy significantly lower than in silica) such as ZBLAN, the 4I11/2 level has a lifetime on the order of 10 ms which is a large fraction of that of the metastable. Furthermore, there is an ESA within the 980 nm pump band which results in the excitation of an ion from the 4I11/2 to the 4I7/2 state and this process can reduce the efficiency of the amplifier. As a result, EDFAs made out of low phonon energy glasses have typically not been pumped in the 980 nm band. Recently, there has been work at finding 980 nm band wavelengths which would provide efficient amplification primarily for obtaining a high inversion to get a low noise figure (good noise performance) (M. Yamada, Y. Ohishi, T. Kanamori, H. Ono, S. Sudo and M. Shimizu, xe2x80x9cLow-noise and gain-flattened fluoride-based Er3+-doped fiber amplifier pumped by 0.97 xcexcm laser diode,xe2x80x9d Optics Letters, vol. 33, pp. 809-810, 1997; M. Yamada, Y. Ohishi, T. Kanamori, S. Sudo and M. Shimizu, xe2x80x9cLow-noise and gain-flattened fluoride-based Er3+-doped fiber amplifier pumped by 0.97 xcexcm laser diode,xe2x80x9d Optics Letters, vol. 22, pp. 1235-1237, 1997). Good power conversion efficiencies have yet to be demonstrated, though, so cost effective 980 nm band pumping has yet to be proven. Since N1+N2 is approximately equal to the total number of active erbium ions in the amplifier, Eqn. 1 can be written in terms of fractional populations as
GdB(xcex)=10 log10(e)xcex93NtotLEDF[{overscore (n2+L )}("sgr"e+"sgr"a)xe2x88x92"sgr"e]xe2x80x83xe2x80x83(3)
where ni=Ni/Ntot. In Eqn. 1, all of the variables on the right side of the equation, except n2, are fixed once the EDF has been manufactured, cut to length and constructed into an amplifier. Therefore, if the gain of the amplifier is to be changed, n2, the average inversion, needs to be changed. However, xcex94GdB/xcex94n2, the change in the gain per xcex94n2 change in inversion, is proportional to xcex93("sgr"e+"sgr"a) which is spectrally dependent. As a result, the gain of a conventional EDFA cannot be increased or decreased in a spectrally uniform manner. Instead, if steps are taken to increase or decrease the gain at a particular wavelength, the shape of the gain spectrum as a function of wavelength will be distorted.
In view of the foregoing, it is a further object of the invention to provide an EFDA in which the gain can be increased or decreased in a spectrally uniform manner. In other words, it is an object of the invention to provide an EFDA or other optical fiber amplifier an overall scale factor of the amplifier gain spectrum may be adjusted substantially independently of the shape of the gain spectrum.
Dual or multiple wavelength (or hybrid) pumping of erbium-doped fiber amplifiers (EDFAs) has been proposed to achieve a number of goals (see U.S. Pat. No. 5,710,659). For example, 980 nm pumping is typically used in the first stage because it can achieve a full inversion of the erbium ions and thus attain the best possible noise performance. On the other hand, 1480 nm pumped gain stages can have a better power conversion efficiency than 980 nm pumped stages (a smaller fraction of the less energetic 1480 nm photons are dissipated within the amplifier in order to generate 1530 nm signal photons) and 1480 nm pump lasers may cost less in some cases. Thus, a two stage amplifier in which the first is pumped with 980 nm and the second is pumped with 1480 nm may combine some of these advantages. This approach could then be extended to a single gain stage by simply pumping it from opposite ends with different pump wavelengths. This provides the added benefit of making it possible to combine multiple pumps.
In contrast to the foregoing, it is a particular objective of the invention to provide an EDFA in which the gain can be increased or decreased in a spectrally uniform manner using multiple pumping wavelengths.
In the present invention, one or more auxiliary xe2x80x9cpumpxe2x80x9d or control wavelengths are used such that some fraction of the dopant ions may be intentionally placed into states other than the ground state or the metastable level such that Ntot greater than N1+N2. In this case, Eqn. (1) can be written as                                           G            dB                    ⁡                      (            λ            )                          =                  10          ⁢                      xe2x80x83                    ⁢                                    log              10                        ⁡                          (              e              )                                ⁢          Γ          ⁢                      xe2x80x83                    ⁢                      L            EDF                    ⁢                                    N              tot                        ⁡                          [                                                                                          N                      1                                        _                                    +                                                            N                      2                                        _                                                                    N                  tot                                            ]                                ⁢                      {                                                                                n                    2                                    _                                ⁡                                  [                                                                                    σ                        e                                            ⁡                                              (                        λ                        )                                                              +                                                                  σ                        a                                            ⁡                                              (                        λ                        )                                                                              ]                                            -                                                σ                  a                                ⁡                                  (                  λ                  )                                                      }                                              (        4        )            
The shape of the gain spectrum in this case is again determined by the relative average inversion n2, but the overall scale factor of the gain spectrum is now proportional to the term fact=[(N1+N2)/Ntot] which is typically unity or not controlled in current EDFAs. However, according to the invention, fat can be brought to less than unity and controlled such that the same gain spectral shape can be preserved at different absolute gain values. In a more general sense, the shape of the amplifier""s gain spectrum can be separated from the absolute gain itself. As a result, an amplifier can be designed to be a flexible xe2x80x9cdynamic gain tiltxe2x80x9d compensator. Its gain shape can be tuned to cancel imbalances arising from dynamic gain tilt in other amplifiers in a cascade. Its absolute gain level could then be adjusted to match the power levels needed by, e.g., link terminal equipment.
One illustrative embodiment of the invention, which implements the foregoing may be described as follows. An optical amplifier comprises an optical fiber segment doped with impurity ions for providing optical gain for an optical signal propagating in the optical fiber segment.
A first source of a first pumping wavelength pumps the ions from a first ground state to a second metastable state. The metastable decays state to the ground by stimulated emission to provide the optical gain. A second source of a second pumping wavelength pumps the ions from the ground state to a third auxiliary state. The auxiliary state decays spontaneously down to the metastable state. Thus, by controlling the pumping power in one or both pumping wavelengths, it is possible to control the fraction of ions in the metastable state. This in turn permits control of an overall scale factor of the gain spectrum without substantially affecting the shape of the gain spectrum.
Illustratively,
(a) the ions are erbium
(b) the first pumping wavelength is 1480 nm
(c) the metastable state is 4I13/2 
(d) the second pumping wavelength is 980 nm, and
(e) the third state is 4I11/2.
Preferably, the optical fiber segment is formed of a low phonon energy glass such as ZBLAN. (Other low phonon energy glasses include tellurites and cesium aluminates). In this case, the lifetime of the auxiliary state is a significant fraction of the lifetime of the metastable state. This makes it possible to control the population of the metastable state and thus the scale factor of the gain spectrum by pumping a fraction of the ions into the auxiliary state.